Fix handling of dirty time zone aware attributes

Previously, when `time_zone_aware_attributes` were enabled, after
changing a datetime or timestamp attribute and then changing it back
to the original value, `changed_attributes` still tracked the
attribute as changed. This caused `[attribute]_changed?` and
`changed?` methods to return true incorrectly.

Example:

    in_time_zone 'Paris' do
      order = Order.new
      original_time = Time.local(2012, 10, 10)
      order.shipped_at = original_time
      order.save
      order.changed? # => false

      # changing value
      order.shipped_at = Time.local(2013, 1, 1)
      order.changed? # => true

      # reverting to original value
      order.shipped_at = original_time
      order.changed? # => false, used to return true
    end

(cherry picked from commit bc982cbcb34129ea2cfe8aa1f8e0b40e444e68db)

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

Backport of #9073
Fixes #8898

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
index 45810322de..6f3112e654 100644
--- a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
+++ b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
@@ -43,9 +43,9 @@ module ActiveRecord
                     time = time.is_a?(String) ? Time.zone.parse(time) : time.to_time rescue time
                   end
                   time = time.in_time_zone rescue nil if time
-                  changed = read_attribute(:#{attr_name}) != time
+                  previous_time = attribute_changed?("#{attr_name}") ? changed_attributes["#{attr_name}"] : read_attribute(:#{attr_name})
                   write_attribute(:#{attr_name}, original_time)
-                  #{attr_name}_will_change! if changed
+                  #{attr_name}_will_change! if previous_time != time
                   @attributes_cache["#{attr_name}"] = time
                 end
               EOV